I did $20 once. The first guy that installed my system five years ago. He did a great job with placing the dish where I can easily get at it, running the lines nice, and leaving plenty of slack like I requested on all the lines.
He was a respectful installer. He asked me if I prefer he removed his shoes, which I did because we just installed new carpet. He answered all my questions thoroughly and knew what he was talking about, and seemed to be pleased to be working at his job. It was a really hot and humid day, so I offered him some lemonade when he came in to finalize everything on the receivers, and when he was done, I gave him $20 for all he had done, and the great attitude he did it with. He really took pride in his work, so I showed him I appreciated it.
Since that first install, I had three other guys out for services or upgrades, and have never tipped since. They all seem to think it is OK to use foul language when talking in general conversation, I had to ask them to remove their shoes, and they treated their job as if it was just a way to pay the bills.
The last one who was out a couple of months ago was installing my HR20, and he was definitely on track to receive a $20 tip. However, he did not remove his shoes, and I did not notice until he was sitting in my Lazy Boy setting up the receiver, and he used foul language when I was engaging in conversation with him. Otherwise he was very nice, and seemed to take pride in his job. It was the foul language and shoes that lost him a tip.
To any installers out there, take note... refrain from foul language, and always ask the customer if they prefer you to remove you shoes!